(One example from the early days of EPA regulation: The last MG Midget sports cars from 1976 to 1979 used the engine and transmission from the Triumph Spitfire sports car also made by the same company, meaning the lighter Midget didn't need to be separately tested.)

Surprise and questions

But few outside Ford realized that Ford had not tested the new C-Max separately; the revelation sparked a number of questions from reporters during the media teleconference.

The company is "now starting to understand hybrid variability" and "anomalies," Ford said on its conference call.

It said it has worked with the EPA, which has been investigating C-Max gas mileage, and concluded that "hybrids have a lot more variability and sensitivity to variables."

Accordingly, the company has now begun testing its C-Max models separately.

Ford noted that it had used the same approach with its C-Max Energi and Fusion Energi plug-in hybrids.

In that case, it tested the C-Max plug-in--which it expects to be the higher-volume vehicle of the pair--and used that 43-mpg rating for the Fusion Energi as well.

Had it tested the plug-in Fusion separately, Ford suggested, that car may have achieved a slightly higher rating on its own.

Industry analyst Dave Sullivan of Autopacific later asked Ford whether it had done the same for non-hybrid vehicles that are in the same weight class and use identical powertrains.

EPA statement: few clues

Many commentators have questioned whether the EPA will revisit the way it measures and certifies fuel economy for hybrid models, in light of Ford's troubles with the C-Max.

One option: The agency could revise the "adjustment factors" for hybrid vehicles.

It's already done that once, in 2007, when it re-rated a number of the earliest hybrids--including the first-generation Honda Insight, the Honda Civic Hybrid, and the Toyota Prius--to compensate for overly high ratings that weren't achieved by owners in real-world use.

A three-page EPA statement on the C-Max rating change offers few clues to what may happen in future.